"till I behold him— dead —
Is my poor heart..."
See in text (Act III - Scene V)
Because of the complicated syntax of this statement, Juliet is able to say two things at once. It can read as "till I behold him dead" meaning she will not rest until she has vengeance. However, it can also read "till I behold him, dead is my poor heart," meaning that her heart is dead over Romeo's absence and she will not rest until she can see him. In this way, Juliet is able to appease her mother and embed her true feelings in her words.